In2.24(NCN)3 and NaIn(NCN)2: Synthesis and Crystal Structures of New Main Group Metal Cyanamides

Abstract

Single crystals of In2.24(NCN)3 and NaIn(N CN)2 can be synthesized from a reaction between InBr and NaCN at 400 °C, followed by chemical transport at 400-500 °C. The X-ray crystal structure determinations of In2.24(NCN)3 (rhombohedral R3̄c, a = 606.09(4), c = 2884.4(2) pm, Z = 6) and NaIn(NCN)2 (orthorhombic Cmcm, a = 961.30(6), b = 716.84(5), c = 603.65(4) pm, Z = 4) reveal ionic layer structures built from sheets of cationic and anionic (cyanamide) motifs. While NaIn(NCN)2 contains In3+ and Na+ ions, In2.24(NCN)3 is a mixedvalence compound, incorporating a monovalent In+ ion besides trivalent indium. The NCN2- units of both In2.24(NCN)3 and NaIn(NCN)2 reflect perfect D∞h point group symmetry within the limits of experimental accuracy. Furthermore, an evaluation of all presently available crystallographic data on metal cyanamide crystal structures manifests that there is no evidence for a non-centrosymmetric description of any of these. Likewise, we propose an alternative centrosymmetric structural model for lead cyanamide, PbNCN, and question the evidence of the unsymmetrical, non-D∞h shaped cyanamide anion in the latter structure.